Steel sheet and strip for use in the manufacture of easy-to-open cans

ABSTRACT

A steel sheet and strip for use in the manufacture of easy-toopen cans having excellent can opening and scoring properties, the compostiion of which is characterized by less than 0.02 percent of carbon, less than 0.1 percent of silicon, 0.1 - 0.5 percent of manganese and less than 0.03 percent of oxygen, with the remainder being iron and unavoidable impurities.

United States Patent 1 Takahasi et a1.

STEEL SHEET AND STRIP FOR USE IN THE MANUFACTURE OF EASY-TO-OPEN CANS Inventors: Nobuyuki Takahasi; Shuji Nagata;

Toshiro Nishihara; Kazuhiko Yoshinari, all of Kitakyushu, Japan Nippon Steel Corporation, Tokyo, Japan Filed: Nov. 5, 1971 Appl. No.: 195,923

Assignee:

Foreign Application Priority Data Nov. 10, 1970 Japan 45/98335 US. Cl 72/365, 72/700, 75/123 D Int. Cl B211) 3/02 Field of Search 148/2, 12.1, 36;

References Cited UNITED STATES PATENTS 1/1969 Fukuda et a1 148/121 Primary Examiner-Lowell A. Larson Attorney-Torch and McGeady [57] ABSTRACT A steel sheet and strip for use in the manufacture of easy-to-open cans having excellent can opening and scoring properties, the compostiion of which is characterized by less than 0.02 percent of carbon, less than 0.1 percent of silicon, 0.1 0.5 percent of manganese and less than 0.03 percent of oxygen, with the remainder being iron and'unavoidable impurities.

2 Claims, No Drawings STEEL SHEET AND STRIP FOR USE IN THE MANUFACTURE OF EASY-TO-OPEN CANS This invention is relates to steel sheets and strips having excellent characteristics for use in the manufacture of the so-called easy open can in which a score with suitable depth is engraved on a can body or cover, and the can can be opened without using a tool, such as a can-opener and a manufacturing method therefore.

In recent years, easy open cans in which the cover of closed vessels, such as, beer cans, carbonated beverage cans, juice cans, glass vessels, and plastics vessels is opened without using a tool, such as a can-opener or cork screw and such covers have been manufactured by using aluminum.

While such a cans and covers will be increasingly used due to the convenience of handling, aluminum cannot be applied for every type of containers because it possesses poor corrosion resistance against certain types of can contents. Thus, the developement of an easy open can using steel sheets and strips has been under investigation. However, since steel is tough, the development of an easy open can using steel sheets and strips which can be opened easily as with aluminum has been difficult to achieve. It has been found by experience that the ease of opening a can be improved by making the rest thickness after score work as thin as possible. On the other hand, however, when the score rest thickness is made thin, cracks are formed at the corner part of the score bottom, thereby partially lowering the strength of the top of the can. Thus, the can can not bear the pressure developed in the can, and the cover will be damaged during transportation and storage.

Accordingly, it is necessary to make the scoreas thin as possible in the range where cracks will not form and the cover is strong enough to avoid trouble during use. However, a fluctuation of about 0.015 mm in the score rest thickness is unavoidable in the presentmanufaeturing techniques, and therefore, for use as a material for covers of easy open cans, the metal should be such that no crack is formed even when the thickness becomes at least 0.03 mm thinner than the maximum score rest thickness normally used. Tin-cans manufactured heretofore are opened with difficulty, regardless of the degree of temper, as the material is tough as above mentioned, and they cannot be opened aseasily as aluminum covers unless the score rest thickness is fairly thin. Presently score rest thickness of tin cans differs at most, only about 0.01 mm from the critical score rest thickness at which point cracks areformed.

In such a situation as above mentioned, the present inventors conducted various experiments and investigations for the purpose of obtaining steel sheets and strips for use in easy open cans having excellent can opening properties and no tendency for crack formation in the score work. This invention is based on the findings that steel sheets and strips comprizing less than 0.02 percent of carbon, less than 0.1 percent of silicon, 0.1 0.5 percent of manganese and lessthan 0.03 percent of oxygen, with the remainder being iron and unavoidable impurities, and further, steel sheets and strips containing 0.04 0.10 percent ofphosphorous besides said component elements are excellent for such purposes.

The steel sheets and strips of this invention differs first of all, as compared with the conventional steel sheets and strips for can use, in the carbon content (0.05 0.08 percent in conventional steel sheets and strips) and the oxygen content (004 0.05 percent in conventional steel sheets and strips).

As referred to herein, the usual manufacturing method of steel sheets and strips for can manufacture means the method whereby molten steel is smelted in a converter, an electric furnace, or by a suitable method, and is made into ingots and cogged to obtain slab, or casted continuously to obtain slab. The slab is then hot rolled at a temperature above 850C and cold rolled with a draft of more than 30 percent. Thereafter decarburizing annealing or the usual annealing is carried out at a temperature of 500 1000C and skin pass rolling is effected with a draft of less than 2.0 percent.

The particular feature of the steel sheet and strip of this invention is that the ease of can opening is improved, depending on the shape of the score (sectional shape and plane shape), when said skin pass is carried out with a draft range of 3 15 percent. The range of this skin pass draft corresponds also to the range where the expanding workability necessary for the steel sheet and strip is not substantially lowered.

The reason why the above component range of the steel sheets and strips for use in the manufacture of easy-to-open cans is important, is as follows:

It is desirous that the carbon content be as low as possible, because the steel becomes hard when the carbon content is larger than 0.020 percent, thus the opening of the can becomes more difficult and cracks are easily formed at the score portion. To reduce the carbon content below 0.020 percent, methods, such as subjecting the molten steel to a vacuum degassing treatment or to adecarburization annealing may be used. The decarburization annealing is more suitable in improving the can opening properties, because it is possible to reduce the carbon content in the steel to extremely low values.

While silicon is an unavoidable component in the steel making process, its content should be restricted below 0.1 percent, because larger amounts thereof makes the steel hard and the can. opening properties become poorer.

Manganese is added in order to prevent the hot shortness of steel material due to the existence of sulphur in steel as anunavoidable component. The existence of 0.1 0.5 percent of manganese is necessary.

When a large amount of oxygen is present, the amount of oxide inclusions increases and cracks are easily formedat the score portion. Therefore, thecontent is desirously as low as possible. However, in this invention, oxygendoes not injure the characteristic of the inventive steel sheet and strip so long as the amount is below 0.03 percent.

The addition of phosphorous remarkably improves the can opening properties of the present steel sheets. The addition of phosphorous is not normally desirable because of.lowered corrosion resistance of the can material. However, in the inventive steel sheets and strips whose carbon content is restricted below 0.020 percent, the addition of 0.04 0.10 percent of phosphorous has almost no effect on the corrosion resistance.

Steel sheets and strips having the above mentioned components and treated by the processes described in this specification, after surface control applications suchas, degreasing and acid pickling and surface treatments such .as, tin-plating, chromium-plating, phos phate treatments or other chemical treatment, may

then be used in the manufacture of easy-to-open cans.

An example of this invention will be stated in the following: Example 1.

Steels with the components as shown in Table l were smelted in a converter. Among them, steels A, B, C and D were decarburized and deoxidized by using a vacuum degassing apparatus until the composition reached a defined range in a molten state. All the steels were, after cogging, hot rolled to reduce the thickness to 2.3 mm at a finishing temperature of 890C and winding temperature of 600C according to the usual strip process. After acid pickling, they were cold rolled to reduce the thickness to 0.24 mm. The cold reduction of a half of hot rolled coil G (G 2) was to 0.26 mm. Then, steels, A, B, C, J and K were annealed at 680C in an atmosphere of H 4 percent and N 96 percent, by using a box annealing furnace. Steels, D, E, F, G, H and I were subjected to decarburization annealing at 700C in an atmosphere of H 40 percent and N 60 percent, by using an open coil annealing furnace. The carbon contents of these steels after decarburization annealing are also shown in Table l. Decarburization annealing of steel I was intentionally suspended halfway through the treatment to give a carbon content of 0.025 percent.

These annealed coils were skin pass rolled with a draft of 1.5 2.0%. Steel G-2 having a thickness of 0.26 mm was skin pass rolled with a draft of 8% to finish the thickness TABLE 1 Chemical component (wt.

to 0.24 mm. The strips were then tin-plated in a conventional electroplating line. Steel strips manufactured by such processes were worked to obtain pull open type can covers, and the forces necessary for can opening when the score rest thickness were, respectively, 0.09 mm and 0.07 mm were measured by using a lnstron tensile tester. The critical score rest thickness without forming cracks at the score part was also investigated by varying the score rest thickness. The results are shown in Table 2.

As obvious from Table 2, the force necessary for can opening is low in the case of steels A, B, C, D, F, G 1 and G 2 made from the steel TABLE 2 (thickness of the material: 0.24 mm) Skin pass Force necessary for can opening of the present invention, and nearly comparable with the force necessary for can opening in the case of a pull open type can cover made of aluminium which is widely used at the present time (manufacturing standard: 3.6 kg in maximum). Their critical score rest thickness without forming cracks at the score part are also very thin. When the upper limit of score rest thickness at which point the can can easily be opened is 0.09 mm, the can can be manufactured smoothly without the formation of cracks, and the fluctuation in the score work by the can making machine being taken into consideration.

The force necessary for can opening is particularly low in steels containing phosphorous (C, F, G and H), thus the effect of the addition of phosphorous can be distinctly observed.

Steels having components without the scope of this invention (H, l, J and K) are defective in that either the force necessary for can opening is high or the critical score rest thickness without forming cracks is too thick. They are obviously inferior to the present steel sheets with respect to the properties required for use in the manufacture of easy open cans.

Steel G 2, which is obtained with a skin pass draft of 8 percent, has similar properties both in the force necessary for can opening and in the critical score rest thickness without forming cracks as in the case of G 1. As already stated, the rigidity of the inventive steel which is skin pass rolled with an ordinary draft has some influence on the force necessary for can opening in some instances according to the plane shape of the score. For the use of covers with such a score shape, the inventive steel sheet and strip skin pass rolled with a draft of 3 15 percent such as steel G 2 is particularly suitable.

As shown in the above example, steel sheets and strips within the component range of this invention are quite excellent for use in the manufacture of easy-toopen cans.

What is claimed is:

l. A steel sheet for use in the manufacture of easy-toopen cans which consists essentially of less than 0.02 percent carbon, less than 0.1 percent silicon, 0.1 0.5 percent manganese, less than 0.03 percent oxygen, 0.04 0.10 percent phosphorous, with the remainder being iron and unavoidable impurities.

2. A method for manufacturing a steel sheet for use in the manufacture of easy-to-open cans which comprises skin pass rolling, with a draft of 3 to 15 percent, a steel sheet obtained by the usual manufacturing method and which consists essentially, after annealing, of less than 0.02 percent carbon, less than 0.1 percent silicon, 0.1 0.5 percent manganese, less than 0.03 percent oxygen, 0.04 0.10 percent phosphorous, with the remainder being iron and unavoidable impurities. 

2. A method for manufacturing a steel sheet for use in the manufacture of easy-to-open cans which comprises skin pass rolling, with a draft of 3 to 15 percent, a steel sheet obtained by the usual manufacturing method and which consists essentially, after annealing, of less than 0.02 percent carbon, less than 0.1 percent silicon, 0.1 - 0.5 percent manganese, less than 0.03 percent oxygen, 0.04 - 0.10 percent phosphorous, with the remainder being iron and unavoidable impurities. 